Life Science

Revealing Same-Sex Attraction’s Evolutionary Role

One researcher brings back a dead hypothesis

January 6, 2010

Paul Vasey is one of the luckiest sexuality researchers working today.

Not only has he secured funding for basic sexuality research — a low priority for most research institutions — but he also gets to do his work on the tropical islands of Samoa.

“Samoa has become a second home to me,” said Vasey, who teaches at the University of Lethbridge in Alberta, Canada. Since 2003, he has spent up to a third of every year on the remote archipelago in the central South Pacific. It’s there he hopes to understand the evolutionary basis for what would appear to be a genetic dead end — same-sex attraction.

With little acknowledged evidence for the existence of homosexuality in nature, and no sound theories to explain its evolutionary purpose, much of society and science have long viewed same-sex attraction as abnormal and deviant. Only in 1973 did the American Psychiatric Association remove homosexuality from its list of mental disorders.

Today, we know homosexual behavior occurs in many animals — from beetles to birds to bison — as well as in humans, but scientists still question how such behavior could have arisen naturally.

Vasey believes Samoa may hold a possible answer. He has found evidence on the tropical islands he frequents to support a hypothesis that same-sex attraction confers an evolutionary advantage to humans by increasing a family’s reproductive rates, as well as ensuring the well-being of future generations.

Studies on families with gay members have repeatedly suggested homosexuality has a genetic component. However, biologists struggle to explain how genes contributing to homosexuality could have stuck around in human evolution. Normally, traits that reduce an individual’s reproductive success, which same-sex attraction would appear to do, would not be maintained in the gene pool.

The kin selection hypothesis, introduced in the 1970s, proposed a possible advantage homosexuality would have for humans. The hypothesis says that while homosexual individuals do not directly pass on their genes to successive generations by having children, they indirectly spread their genes through their families.

By devoting their energy to raise their nieces and nephews instead of having children of their own, homosexuals would allow their siblings to have more children and ensure that these offspring live to have children of their own. Thus, homosexual individuals would promote greater reproductive and survival rates of the relatives who share many identical genes with them — including those that may contribute to homosexuality — guaranteeing these genes are passed on to future family members.

Though seemingly logical to many researchers, the idea lost credibility as European and North American studies failed to find evidence that gays devote a significant amount of time and energy to caring for their siblings’ children. Vasey decided to test the hypothesis somewhere else. To eliminate influences Western culture may have had on previous studies, Vasey studies the faʻafafine, feminine Samoan males who reject the labels of “gay” and “homosexual” but acknowledge their primary sexual attraction to adult men.

Vasey is also interested in the Samoans because their social environment closely resembles the one in which our ancestors evolved. “Individuals in the family are interconnected in ways that don’t characterize European society anymore,” Vasey said. “Samoans remain close to their family in ways that characterize the evolutionary past.” Also, Samoan society fully accepts the faʻafafine, who Vasey said are considered to occupy a third gender and are respected for their commitment to taking care of their families.

Vasey analyzed how Samoan men, women and faʻafafine acted toward their nieces, nephews and unrelated children living in the same village, looking at behavior and attitudes such as their willingness to babysit and contribute financial resources. In results to be published in the February 2010 issue of Psychological Science, Vasey found the faʻafafine were more willing to help their nieces and nephews and less likely to help other children compared to their straight counterparts, even when controlling for age, income levels and number of biological children.

Though not conclusive, these results fall in line with the kin selection hypothesis. “You would expect there to be selection to make faʻafafine as efficient as possible in terms of channeling resources and energy toward nieces and nephews without having resources and energy diverted toward non-kin,” Vasey said.

Cognitive biologist Qazi Rahman from the Queen Mary, University of London believes Vasey’s work has revitalized the kin selection hypothesis. “I didn’t think it had any use anymore,” he said. “But now the science is different.”

Nevertheless, some experts caution against jumping to conclusions based on Vasey’s results. “While his science is strong, a lot more has to be done to convince me,” said Northwestern University psychologist Michael Bailey, who found no evidence supporting kin selection in a 2001 study with American homosexual males. “The math still isn’t there.”

Also, while this work might explain the prevalence of gay men, evidence of evolutionary pathways for female homosexuality has proven elusive, as female sexuality seems “more fluid,” and harder to categorize than male sexuality, according to Vasey.

Next year, Vasey plans to further test the kin selection hypothesis by studying faʻafafine avuncularity, or uncle-like behavior, and its measurable effects on children. He also plans to investigate whether Samoans “just expect faʻafafine to behave in a more avuncular manner.” However, Vasey stresses that a cultural expectation does not “invalidate the evolutionary mechanism,” as biological and cultural influences most likely work in concert.

Vasey, who is gay, understands that some may use his sexuality to discount his results. Nevertheless, he stands by his work.

“We’ve just found over and over again consistent evidence in support of these evolutionary pathways,” Vasey said. “When I first started, I thought we’d do one study on this, the results would be negative, and that would be the end of it. No one is more surprised by the results than me.”

*Correction (January 11, 2010): This sentence originally read: “Only since 1973 has the American Psychological Association removed homosexuality from its list of mental disorders.”

**Correction (January 13, 2010): This sentence originally read: “By providing support to raise their nieces and nephews instead of having children of their own, homosexuals would allow their siblings to have more children and ensure that these offspring live to have children of their own.”

About the Author

Alex Liu

A Bay Area native, Alex Liu studied toxicology at the University of California, Berkeley and then spent three years developing oncology medication at Genentech. Currently attending New York University’s Science, Health and Environmental Reporting Program, he hopes to bridge the gap between science and public policy. He’s interned with NOVA scienceNOW and CNN’s medical unit, and loves working on all things video. In his free time he enjoys powering through seasons of television shows, traveling, rooting for Oakland sports teams, and stepping out onto the dance floor. You can visit his personal website and follow him on Twitter.



yar2009 says:

Suppose there to be incontrovertible evidence that the kin selection hypothesis is true, what have we learned? Perhaps we might realize then that such a pathway to the fostering of genes that lend themselves to ‘gayness’ are not desirable. The process by which this activity has prospered in the past might be purposely disrupted so as to vitiate such a malignant outcome.

Alice Dreger says:

@1 Or maybe nothing will come of it, or maybe lots of folks will decide, with the naturalistic fallacy, that because it is “biological” then it is fine. You should read this by Tim Murphy:
Murphy there concludes: “…all things considered, it is better to approach human sexuality in full knowledge of its nature (so far as we can know it) than to presume that we already know enough, or that we already do enough, to protect men, women, and children from social mistreatment rooted in scientific half truths.”

Joe Marfice says:

OK, I think his research may be valuable… unlike the pandering quality of this article. “Only since 1973 has the American Psychological Association removed homosexuality from its list of mental disorders.” Only? For 36 years? Long enough for a child to have been born, grown to maturity, voted in an election, produced a baby, which then grew to maturity, and voted in an election… ONLY that long?

There are plenty of injustices in this world. Events that happened three decades ago aren’t really, well, timely to this article’s focus.

ed says:


the last civil rights case involving interracial marriage was in the 80s. A WHOLE TWO DECADES AGO. that’s enough time for me to be born and be old enough to know that the point of that sentence was to illustrate how long homosexuality has been misunderstood vs citing some kind of wrongdoing by indicating the length of time as the injustice. also, 36 years means that every person who is 50+ years old remembers the times when it was “rightfully” labeled a mental disorder.

do you think that has no impact whatsoever?

it’s called context. protip: you give it in news articles.

C.R. says:

There is no evolutionary role, or genetic component in homosexual behavior. Insects, birds and other animals do not engage in homosexual behavior–homosexual behavior is a human phenomenon. [Human beings are not animals!] Paul Vassey is no scientist.

Raymond Hames says:

Contrary to what CR claims homosexuality is widespread among non-humans: See “Biological Exuberance: Animal Homosexuality and Natural Diversity” by Bruce Bagemihl. But what can you expect from someone who says “Human Beings are not animals!”

By the way, Vasey is an excellent scientist.

L.R says:

Well said Raymond. Very interesting research!

Christine says:

What an absurd statement. There is lots of evidence of homosexuality amongst animal species. Mr Vassey is not a scientist, he is a researcher as the article clearly states.

As for 30+ years since homosexuality was removed as a disorder, not a lot has changed, people are still tormenting, torturing, killing homosexuals for just being human and doing no harm to others. The American and Canadian Psychiatrist Associations have removed this disorder (how nice of them, doesn’t mean the rest of the world has moved forward in any way.

Michael says:

Interesting article. I’d have to say I find it makes a great deal of sense, there are people out there with little to no sexual interest for example who this would also be a fitting explanation as to their continuing genes.

C.R.’s commentary is typical of those of an extremist religious opinion so I must agree, well said!

Agreed, if people practice the same love of others of their kin regardless of sexuality, the world might just be that bit more perfect.

JonnyBoy says:

There is research to suggest that female relatives of gay men have more children. This has lead to a theory that the genes that improve fertility in females cause homosexuality in males – as a kind of side effect.

But this theory may also be true – not for the idea that gay men take care of children – just that there may be a another good reason that a percentage of the population don’t breed . . .

Kathy Mulholland-Isabell says:

All research exploring aspects of same-gender relationships and attraction in various cultures cannot but help in our understanding of the marvelous spectrum of diversity that is the human condition.

Only earlier today I wondered about marriage, and what role models I would like children to have. Would I want children to model Hank and Susan down the street, who fight, frighten their kids (one of whom confided to me that truck-driver dad is a “hustler” who drives around the country selling cocaine) and frequently have the police called to their home? Or would I rather have them model my friends Leon and Gerry, who hug every time they see each other after the briefest of absences, have raised their three sons successfully to young adulthood (all three are in college and doing well), and who are themselves well-respected professionals — Leon is a social worker working with at-risk urban kids, has been doing so for over a decade? My answer is probably obvious.

I hope that more studies continue to debunk any notions that children and families need anything more than loving, stable parents and other relatives, regardless of the genders and orientations of those parents and relatives.

reverberation22 says:

This is well-written! I think homosexuality should be encouraged ;D

Amber says:

I have some issues for this article. There is mention that homosexuals do not have children. Although they can’t biologically produce children AS A COUPLE, many homosexual couples are having children via artificial insemination. As a result, a lesbian couple can give birth to their OWN children and raise them together. Some lesbians will have children with the SAME sperm donor so that their children will be half-siblings. Also, many gay men are having children with the same egg donor, so that they’re children will be half-siblings, and they raise them together.

The world hasn’t changed much in terms on their views on homosexuality. People still treat it as if it is a disease. People torment homosexuals worldwide, and think something is wrong with them. This world is still homophobic.

Ian says:

The problem with this kind of thing is that we 1. try to generalise something that can’t be generalised. Sexuality is as diverse as the number of individuals practicing it. 2. We are thinking too small. Imagine what this world would look like, it everybody had children. We should be grateful for all the reasons nature has found to limit the reproductive capacities of humans. Be it ethical, sexual, religious, medical or mental. The answer to why homosexuality has survived in the human gene pool, is probably found when looking at humanity as a complete organism inhabiting this planet and what it needs to be able to survive as a whole. Wonder how much time Vasey spends with his nieces and nephews with all that traveling his is doing ……

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